1. Field of the Invention
The invention relates in general to an over current protection circuit and a power converter using the same, and more particularly to an over current protection circuit with a soft start function and a power converter using the same.
2. Description of the Related Art
FIG. 1 (Prior Art) is a schematic illustration showing a first conventional power converter 10(1). Referring to FIG. 1, the conventional power converter 10(1) for providing power Vo to a load 20 includes a switching converter 110, a current sensing circuit 130, an operational amplifier 144, a conventional soft start circuit 142(1), a load sensing circuit 1212, an error amplifier 1214, a compensator 1216, an operational amplifier 1222, a logic gate 1224, a clock generator 1226, a latch 1228 and a buffer amplifier 1229.
The current sensing circuit 130 senses an inductance current of the switching converter 110 to output a current sensing signal a2. The operational amplifier 144 outputs an over current signal a6 according to the current sensing signal a2 and a direct current (DC) level Vlim. The load sensing circuit 1212 senses a current of the load 20, and outputs a feedback signal a1 via the error amplifier 1214 and the compensator 1216. The operational amplifier 1222 outputs a signal a7 according to the current sensing signal a2 and the feedback signal a1, and the OR gate 1224 outputs a signal a4 according to the over current signal a6 and the signal a7. The clock generator 1226 outputs a clock signal a5 to the latch 1228 so that the latch 1228 outputs a signal a3 according to the signal a4 and the clock signal a5. The buffer amplifier 1229 buffers and amplifies the signal a3 into a driving signal DRV and then outputs the driving signal DRV.
The conventional soft start circuit 142(1) converts a reference voltage Vref into a reference voltage Vr and then outputs the reference voltage Vr to the error amplifier 1214, and the conventional soft start circuit 142(1) controls the rising speed of the reference voltage Vr to adjust the duty cycle of the driving signal DRV. However, this system needs the additional soft start circuit to achieve the soft start function.
FIG. 2 (Prior Art) is a schematic illustration showing a second conventional power converter 10(2). Referring to FIG. 2, the difference between the conventional power converter 10(2) and the conventional power converter 10(1) is that the conventional power converter 10(2) provides a signal with a duty cycle, varying from short to long, to the buffer amplifier 1229 so that the duty cycle of the driving signal DRV progressively increases. However, this design has the complicated circuit, and occupies the larger area.